This invention relates to radio broadcasting, and more particularly, to AM In-Band-On-Channel (IBOC) Digital Audio Broadcasting (DAB), and signal processing in AM IBOC DAB transmitters.
Digital Audio Broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. AM IBOC DAB can be transmitted in a hybrid format where it coexists with the AM signal, or it can be transmitted in an all-digital format where the removal of the analog signal enables improved digital coverage with reduced interference. IBOC requires no new spectral allocations because each DAB signal is simultaneously transmitted within the same spectral mask of an existing AM channel allocation. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners.
U.S. Pat. No. 5,588,022 discloses a hybrid AM IBOC broadcasting method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel that includes the steps of broadcasting an amplitude modulated radio frequency signal having a first frequency spectrum, wherein the amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal, and simultaneously broadcasting a plurality of digitally modulated carrier signals within a bandwidth which encompasses the first frequency spectrum, each of the digitally modulated carrier signals being modulated by a portion of a digital program signal. A first group of the digitally modulated carrier signals lie within the first frequency spectrum and are modulated in-quadrature with the first carrier signal. Second and third groups of the digitally modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal.
In AM IBOC DAB systems, frequency domain side lobe constraints together with symbol rate and sub-carrier spacing requirements can lead to signal pulse trains with overlapping pulses. The AM transmission comprises a train of orthogonal frequency division multiplexed (OFDM) pulses. The pulses are made up of evenly spaced sub-carriers. The digitized data is subdivided into xe2x80x9cmxe2x80x9d bit words, converted to amplitude and phase values and then assigned to the sub-carriers. The shape of the pulses is selected so that the sub-carriers are orthogonal to one another when matched filtering is applied at the receiver. In this way, matched filtering can recover the amplitude and phase information for each individual sub-carrier and thereby recover the value of each digital word.
One way of ensuring the required orthogonality is to use rectangular pulses whose duration is the reciprocal of the sub-carrier spacing. The use of non-overlapping rectangular pulses has the desirable feature of maximizing the transmission rate. The main drawback, however, is excessive side lobe levels. Moreover, achieving the desired orthogonality requires exact frequency centering.
The problem of preserving orthogonality while reducing side lobe levels has a time domain dual that was the subject of a classic paper by Nyquist (Nyquist, H., xe2x80x9cCertain Topics in Telegraph Transmission Theory,xe2x80x9d Trans. Am. Inst. Electr. Eng., vol. 47, April 1928, pp. 617-644). The solution is to extend the length of the rectangular pulse and apply a raised cosine weighting to the result. The transmitter and receiver split the weighting with the transmitter and receiver each applying the square root of the weights.
The waveform used in one AM digital audio broadcasting system is the convolution of a Nyquist type pulse with the Gaussian density function. This construction guarantees that the frequency domain side-lobes meet spectral mask requirements imposed by the FCC. The length of the Nyquist pulse is one OFDM symbol period. Convolution increases the pulse length. As a result, the pulses in the transmitted pulse train overlap. The presence of this overlap introduces distortion in the output of the demodulator.
The distortion caused by pulse overlap has an effect similar to that of noise; i.e. demodulator outputs are displaced from their assigned constellation locations. When a large number of demodulator outputs are superimposed on a graph, they give a fuzz-like appearance to the demodulated signal constellation.
It would be desirable to reduce distortion caused by pulse overlap. This invention seeks to provide a method for pre-compensating signal pulses of an AM IBOC digital audio broadcasting system to reduce distortion.
This invention provides a method of pre-compensating at the transmitter for pulse overlap in a digitally modulated signal comprising the steps of receiving a sequence of pulses, modulating the pulses to produce a first sequence of modulated pulses, demodulating the first sequence of modulated pulses to produce a first sequence of demodulated pulses, combining the first sequence of demodulated pulses with the first sequence of pulses to produce a first sequence of error pulses, modulating the first sequence of error pulses to produce a first sequence of modulated error pulses, and combining the first sequence of modulated error pulses with the first sequence of modulated pulses to produce a first sequence of compensated pulses.
The invention further encompasses a method of pre-compensating for pulse overlap in a digitally modulated signal comprising the steps of receiving a sequence of pulses, modulating the pulses to produce a sequence of modulated pulses, storing the modulated pulses, using non-consecutive pairs of the modulated pulses to produce a sequence of error first order terms, storing the sequence of first order error terms, and subtracting each of the first order error terms from corresponding ones of the modulated pulses to produce a first compensated signal.
Transmitters that process signals in accordance with the above methods are also included.